Transceivers are in general utilized to effect a bi-directional communication path between two remote points over a communication channel. The transmission path or line provides the media for the data exchange between these two remote points or stations. This exchange typically includes session establishment, user messages and session termination, where a session is defined as the communication flow between two nodes and a network. The transmission path of the network can take many forms. The path can be a point-to-point or a multi-point configuration utilizing a simplex, half-duplex or duplex transmission format. The transmission format is a way in which message traffic flows across the transmission path.
In simplex transmission, provision is made only for unidirectional transmission. The sender cannot receive and the receiver cannot send. By comparison, half-duplex transmission provides movement of data across the line in both directions, but in only one direction at a time. Typically, a message sent to one terminal requires deciphering of the message, entering of the appropriate response and sending the reply, i.e. handshaking. The users on either end of the line take turns utilizing the line. Duplex transmission (also referred to as full-duplex) provides for simultaneous two-way transmission between the two stations. A duplex transmission permits the interleaving of sessions and user data flow among several or many stations.
In most transceiver systems utilized for communicating between two remote points, full-duplex transmission is generally required. One type of system providing for this type of transmission is that utilizing a universal digital-loop transceiver, manufactured by Motorola, Part No. MC-145422 and 145426. In this type of system, a single twisting pair cable is provided which may be up to two kilometers in length. These transceivers are primarily designated for use in digital subscriber voice/data telephone systems or in remote data acquisition and control systems. These systems utilize a "ping-pong" data transmission scheme wherein one station acts as a master and one station acts as a slave with both stations having an operating cycle comprised of a transmission frame and a receive frame. Synchronization of the operating cycles is necessary to allow the master to transmit during the receive portion of the slave operating cycle, and likewise, transmission occurs between the slave and master during the receive frame of the master operating cycle. In this manner, transmission of data from the master to the slave does not collide with the transmission from the slave to the master.
There are a number of problems associated with initial synchronization of most transceiver systems. It can be appreciated that once the systems are synchronized, transmission can be effected in accordance with the "ping-pong" transmission scheme. However, when the two transceivers are not synchronized, two transmissions can occur at the same time over the communication channel. Since a separate clock is not transmitted between the two transceivers, the local receiver clock or time base must be synchronized with the received data. However, this necessitates one of the two transceivers being designated a master and one being designated a slave. Ordinarily, the transceiver circuitry of the prior art makes provisions for predesignation of one side of the transmission path as a master and one side as a slave.
A further problem that exists with some transceiver systems is accommodating transmission delay. When the slave side of the transmission system receives transmitted data and locks thereto during a receive frame, it is important that transmitted data from the slave arrive at the master during the receive frame. This synchronization can be effected by ensuring that there is a relatively long delay provided between transmission and reception frames in either the master or the slave, but this method can be uneconomical from a time and speed standpoint. Time and speed become especially critical when utilizing optical systems that are operable to transmit over a long haul data line. The disadvantages with the above systems arise from the fact that the master/slave designation requires two separate chips or provision for external selection therefore, as well as the need for schemes to accommodate the transmission delay.